I've done a fair amount of thinking about the long term effects of HFT's on the market microstructure.
The HFT's reduce the margins to a point where most human traders cannot compete, for the very short term strategies like scalps and liquidity provision... the humans that stick around are mostly trading on longer time frames (may be a short as 5 minutes, but not many are still in the few seconds range) ...
so when things go wrong and the HFT's pullback from their trading, there's no significant volume coming from humans to maintain sufficient liquidity... and you end up getting the flash crash and similar scenarios...
This is only aggravated by regulators that heavily favor HFT... e.g. these algos post orders for < 1 second and cancel >90% of them, and regulators have no problem with this... if human traders post orders for < 1 minute and cancel > 50% of them, they may get in trouble...
The HFT's reduce the margins to a point where most human traders cannot compete, for the very short term strategies like scalps and liquidity provision... the humans that stick around are mostly trading on longer time frames (may be a short as 5 minutes, but not many are still in the few seconds range) ...
so when things go wrong and the HFT's pullback from their trading, there's no significant volume coming from humans to maintain sufficient liquidity... and you end up getting the flash crash and similar scenarios...
This is only aggravated by regulators that heavily favor HFT... e.g. these algos post orders for < 1 second and cancel >90% of them, and regulators have no problem with this... if human traders post orders for < 1 minute and cancel > 50% of them, they may get in trouble...